Message folders which store their data in one single file per message are very inefficient for producing subject overviews and for computing message threads. The Mail::Box::MH::Index object is able to store and read a the headers of a set of Mail::Box::MH::Message messages which are part of a single Mail::Box::MH folder in one file.

When the Mail::Box::MH::Index functionality is enabled by specifying Mail::Box::MH::new(keep_index) when opening a folder, the index file is automatically read. When the folder is closed, a new index file is created.

Special care is taken to avoid problems which occur when the user changes or removes message files without updating the index. If the index is not trustworthy it will not be used (costing some performance for the reader of the folder).

The type of headers which will be used to store header information when it is read from the index file. You can not be sure the index contains all header line (the mailbox may have been updated without updating the index) so this will usually be (an sub-class of) Mail::Message::Head::Subset.

Look if there is header info for the specified $msgfile. The filename represents one message in folder type which are organized as directory. This method will return an object of the new(head_type) as specified during creation of the index object, or undef if the information is not known or not trustworthy -i.e. the file size changed.

Fatal error: the specific package (or one of its superclasses) does not implement this method where it should. This message means that some other related classes do implement this method however the class at hand does not. Probably you should investigate this and probably inform the author of the package.